The Royal Arch Trail is more difficult trail, because the elevation rise of over 1600 feet! Although it's a steeper climb, the trail isn't too long (around 3.5 mile loop). This trail is probably the most popular, because of the beautiful arch at the top! With views overlooking the city, it's a great reward to a tougher climb.
The First and Second Flatiron Trail loop is also a more difficult one. The trail is shorter (2.7 mile loop) and the elevation is lower (over 1400 feet), but the trail is also heavily trafficked. It becomes very rocky at points, and you pretty much climb over rocks to get to the top of the flatirons. It's a workout to get to the top, but a lot of fun!
There are several other trails to try here, but if you don't mind a challenge and want the best of the park, try these!